NRF2 (NF-E2 related factor 2) is a member of the cap-n-collar (CNC) family of transcription factors containing a characteristic basic-leucine zipper motif. Under basal conditions, NRF2 levels are tightly controlled by the cytosolic actin-bound repressor, KEAP1 (Kelch-like ECH associating protein 1), which binds to NRF2 and targets it for ubiquitylation and proteasomal degradation via the CuI3-based E3-ubiquitin ligase complex. Under conditions of oxidative stress, DJ1 (PARK7) is activated and stabilizes NRF2 protein by preventing NRF2 from interacting with KEAP1. Also, modification of reactive cysteines on KEAP1 can cause a conformational change in KEAP1 that alters NRF2 binding and promotes NRF2 stabilization. Thus, the levels of NRF2 in the cytosol are low in normal conditions but the system is designed to respond immediately to environmental stress by increasing NRF2 activity.
Inappropriately low NRF2 activity in the face of on-going oxidative stress appears to be a pathological mechanism underlying chronic obstructive pulmonary disease (COPD). This may be a result of an altered equilibrium between NRF2 regulators with both inappropriate lack of positive regulators such as DJ1, and overabundance of negative regulators such as Keap1 and Bach1. Therefore, restoration of NRF2 activity in the lungs of COPD patients should result in repair of the imbalance and mitigation of deleterious processes such as apoptosis of structural cells (including alveolar epithelial and endothelial cells) and inflammation. The results of these effects would be enhanced cytoprotection, preservation of lung structure, and structural repair in the COPD lung, thus slowing disease progression. Therefore, NRF2 modulators may treat COPD (Boutten, A., et al. 2011. Trends Mol. Med. 17:363-371) and other respiratory diseases, including asthma and pulmonary fibrosis (Cho, H. Y., and Kleeberger, S. R. 2010. Toxicol. Appl. Pharmacol. 244:43-56).
An example of inappropriately low NRF2 activity is found in pulmonary macrophages from COPD patients. These cells have impaired bacterial phagocytosis compared with similar cells from control patients, and this effect is reversed by the addition of NRF2 activators in vitro. Therefore, in addition to the effects mentioned above, restoration of appropriate NRF2 activity could also rescue COPD exacerbations by reducing lung infections. This is demonstrated by the NRF2 activator, Sulforaphane, which increases the expression of Macrophage Receptor with Collagenous structure (MARCO) by COPD macrophages and alveolar macrophages from cigarette smoke-exposed mice, thereby improving in these cells bacterial phagocytosis (Pseudomonas aeruginosa, non-typable Haemophilus influenzae) and bacterial clearance both ex vivo and in vivo. (Harvey, C. J., et al. 2011. Sci. Transl. Med. 3:78ra32).
The therapeutic potential of targeting NRF2 in the lung is not limited to COPD. Rather, targeting the NRF2 pathway could provide treatments for other human lung and respiratory diseases that exhibit oxidative stress components such as chronic asthma and acute asthma, lung disease secondary to environmental exposures including but not limited to ozone, diesel exhaust and occupational exposures, fibrosis, acute lung infection (e.g., viral (Noah, T. L. et al. 2014. PLoS ONE 9(6): e98671), bacterial or fungal), chronic lung infection, al antitrypsin disease, and cystic fibrosis (C F, Chen, J. et al. 2008. PLoS One, 2008; 3(10):e3367).
A therapy that targets the NRF2 pathway also has many potential uses outside the lung and respiratory system. Many of the diseases for which an NRF2 activator may be useful are autoimmune diseases (psoriasis, IBD, MS), suggesting that an NRF2 activator may be useful in autoimmune diseases in general.
In the clinic, a drug targeting the NRF2 pathway (bardoxolone methyl) has shown efficacy in diabetic patients with diabetic nephropathy/chronic kidney disease (CKD) (Aleksunes, L. M., et al. 2010. J. Pharmacol. Exp. Ther. 335:2-12), though phase III trials with this drug in patients with the most severe stage of CKD were terminated. Furthermore, there is evidence to suspect that such a therapy would be effective in sepsis-induced acute kidney injury, other acute kidney injury (AKI) (Shelton, L. M., et al. 2013. Kidney International, June 19. doi: 10.1038/ki.2013.248), and kidney disease or malfunction seen during kidney transplantation.
In the cardiac area, bardoxolone methyl is currently under investigation in patients with Pulmonary Arterial Hypertension and so a drug targeting NRF2 by other mechanisms may also be useful in this disease area. Oxidative stress is increased in the diseased myocardium, resulting in accumulation of reactive oxygen species (ROS) which impairs cardiac function [Circ (1987) 76(2); 458-468] and increases susceptibility to arrhythmia [J of Mol & Cell Cardio (1991) 23(8); 899-918] by a direct toxic effect of increased necrosis and apoptosis [Circ Res (2000) 87(12); 1172-1179]. In a mouse model of pressure overload (TAC), NRF2 gene and protein expression is increased during the early stage of cardiac adaptive hypertrophy, but decreased in the later stage of maladaptive cardiac remodeling associated with systolic dysfunction [Arterioscler Thromb Vasc Biol (2009) 29(11); 1843-1850; PLOS ONE (2012) 7(9); e44899]. In addition, NRF2 activation has been shown to suppress myocardial oxidative stress as well as cardiac apoptosis, fibrosis, hypertrophy, and dysfunction in mouse models of pressure overload [Arterioscler Thromb Vasc Biol (2009) 29(11); J of Mol & Cell Cardio (2014) 72; 305-315; and 1843-1850; PLOS ONE (2012) 7(9); e44899]. NRF2 activation has also been shown to protect against cardiac I/R injury in mice [Circ Res (2009) 105(4); 365-374; J of Mol & Cell Cardio (2010) 49(4); 576-586] and reduce myocardial oxidative damage following cardiac I/R injury in rat. Therefore, a drug targeting NRF2 by other mechanisms may be useful in a variety of cardiovascular diseases including but not limited to atherosclerosis, hypertension, and heart failure (Oxidative Medicine and Cellular Longevity Volume 2013 (2013), Article ID 104308, 10 pages), acute coronary syndrome, myocardial infarction, myocardial repair, cardiac remodeling, cardiac arrhythmias, heart failure with preserved ejection fraction, heart failure with reduced ejection fraction and diabetic cardiomyopathy.
A drug activating the NRF2 pathway could also be useful for treatment of several neurodegenerative diseases including Parkinson's disease (PD), Alzheimer's disease (AD), amyotrophic lateral sclerosis (ALS) (Brain Res. 2012 Mar. 29; 1446:109-18. 2011.12.064; Epub 2012 Jan. 12) and multiple sclerosis (MS). Multiple in vivo models have shown that NRF2 KO mice are more sensitive to neurotoxic insults than their wild-type counterparts. Treatment of rats with the NRF2 activator tert-butylhydroquinone (tBHQ) reduced cortical damage in rats in a cerebral ischemia-reperfusion model, and cortical glutathione levels were increased in NRF2 wild-type but not KO mice after administration of tBHQ (Shih, A. Y., et al. 2005. J. Neurosci. 25: 10321-10335). Tecfidera™ (dimethyl fumarate), which activates NRF2 among other targets, is approved in the U.S. to treat relapsing-remitting multiple sclerosis (MS). Activation of NRF2 may also help treat cases of Friedreich's Ataxia, where increased sensitivity to oxidative stress and impaired NRF2 activation has been reported (Paupe V., et al, 2009. PLoS One; 4(1):e4253.
There is preclinical evidence of the specific protective role of the NRF2 pathway in models of inflammatory bowel disease (IBD, Crohn's Disease and Ulcerative Colitis) and/or colon cancer (Khor, T. O., et al 2008. Cancer Prev. Res. (Phila) 1:187-191).
Age-related macular degeneration (AMD) is a common cause of vision loss in people over the age of 50. Cigarette smoking is a major risk factor for the development of non-neovascular (dry) AMD and perhaps also neovascular (wet) AMD. Findings in vitro and in preclinical species support the notion that the NRF2 pathway is involved in the anti-oxidant response of retinal epithelial cells and modulation of inflammation in pre-clinical models of eye injury (Schimel, et al. 2011. Am. J. Pathol. 178:2032-2043). Fuchs Endothelial Corneal Dystrophy (FECD) is a progressive, blinding disease characterized by corneal endothelial cells apoptosis. It is a disease of aging and increased oxidative stress related to low levels of NRF2 expression and/or function (Bitar, M. S., et al. 2012. Invest Ophthalmol. Vis. Sci., Aug. 24, 2012 vol. 53 no. 9 5806-5813). In addition, an NRF2 activator may be useful in uveitis or other inflammatory eye conditions.
Non-alcoholic steatohepatitis (NASH) is a disease of fat deposition, inflammation, and damage in the liver that occurs in patients who drink little or no alcohol. In pre-clinical models, development of NASH is greatly accelerated in KO mice lacking NRF2 when challenged with a methionine- and choline-deficient diet (Chowdhry S., et al. 2010. Free Rad. Biol. & Med. 48:357-371). Administration of the NRF2 activators oltipraz and NK-252 in rats on a choline-deficient L-amino acid-defined diet significantly attenuated progression of histologic abnormalities, especially hepatic fibrosis (Shimozono R. et al. 2012. Molecular Pharmacology, 84:62-70). Other liver diseases that may be amenable to NRF2 modulation are toxin-induced liver disease (e.g., acetaminophen-induced hepatic disease), viral hepatitis, and cirrhosis (Oxidative Medicine and Cellular Longevity Volume 2013 (2013), Article ID 763257, 9 page).
Recent studies have also begun to elucidate the role of ROS in skin diseases such as psoriasis. A study in psoriasis patients showed an increase in serum malondialdehyde and nitric oxide end products and a decrease in erythrocyte-superoxide dismutase activity, catalase activity, and total antioxidant status that correlated in each case with disease severity index (Dipali P. K., et al. Indian J Clin Biochem. 2010 October; 25(4): 388-392). Also, an NRF2 modulator may be useful in treating the dermatitis/topical effects of radiation (Schifer, M. et al. 2010. Genes & Devl. 24:1045-1058); and The Immunosuppression due to Radiation Exposure, Kim, J. H. et al, J. Clin. Invest. 2014 Feb. 3:124(2):730-41).
There are also data suggesting that an NRF2 activator may be beneficial in preeclampsia, a disease that occurs in 2-5% of pregnancies and involves hypertension and proteinuria (Annals of Anatomy—Anatomischer Anzeiger Volume 196, Issue 5, September 2014, Pages 268-277).
Preclinical data has shown that compounds with NRF2 activating activity are better at reversing high altitude-induced damage than compounds without NRF2 activity, using animal and cellular models of Acute Mountain Sickness (Lisk C. et al, 2013, Free Radic Biol Med. October 2013; 63: 264-273.)